1. Field of the Invention
The present invention relates to computer network connectivity and wireless networking, and particularly, to a cellular wireless access infrastructure, where individual client wireless devices use a set of access point devices as gateway nodes to connect to a wired backbone network.
2. Description of the Prior Art
Wireless access networks are often designed in a cellular fashion, with multiple access points (also called base stations) offering multiple points of attachment to a backbone (e.g., wired) networking infrastructure. Individual wireless client computing devices may connect to the network by attaching to one (or, at least in theory, more) of these access points, as long as they lie within a communication range of the concerned access point(s). The access points (APs) are laid out in cellular fashion, with each access point's area of coverage being defined by an area known as the cell size. The composite architecture thus consists of multiple cells. Since cells can overlap with one another, a wireless client device located inside the region of overlap may have multiple candidate APs, and hence, may be associated with more than one access point. On the other hand, if the cells are non-overlapping, a wireless device has only one candidate access point. Thus, it stands that the set of candidate access points for a wireless devices is a function of, and can be altered merely by changing, the coverage areas of the individual APs.
The coverage area of a cell is principally dependent on the transmission power employed by the AP—as it is a well accepted principle of wireless communications that the larger the transmission power, the greater the communication range and hence, the larger the coverage area, also called the footprint, of the associated cell. Communication disruption and collisions between neighboring cells are avoided through a variety of techniques, including frequency partitioning (such that neighboring cells communicate on non-overlapping frequency bands), code partitioning (such that neighboring cells use orthogonal codes to avoid mutual interference) or time partitioning (such that neighboring cells communicate at non-overlapping time intervals).
Now, each cell (or equivalently, the AP) has a predefined traffic capacity, defined in terms of various parameters such as the available bandwidth or number of channels. If a larger number of wireless devices happen to attach to a single AP, or if some of the attached wireless devices generate excessively large quantities of traffic, that access point can experience congestive overload, leading to either service denial or service degradation. Since the physical movement pattern of wireless devices, or the density of such devices in various physical regions, cannot be predicted in advance, the access points should have a mechanism for dealing with sudden increases or decreases in load. This family of mechanisms is called load-balancing, and can typically take one of two approaches:                Capacity Adaptation: An overloaded access point can try to increase its own capacity by borrowing capacity from neighboring APs.        Load Adaptation: An overloaded access point can try to reduce its own load by forcing or directing some or all of its associated wireless devices to switch to an alternative neighboring AP.        
Most current load-balancing in cellular networks use the Capacity Adaptation solution: an overloaded access point borrows excess capacity from neighboring underutilized APs. Examples of such adaptive capacity adjustment solutions can be found in the works of: S. Tekinay and B. Jabbari entitled “Handover and Channel Assignment in Mobile Cellular Networks” IEEE Communication Magazine, November 1991; S. Das, S Sen and R. Jayaram entitled “Dynamic load Balancing Strategy for Channel Assignment using Selective Borrowing in Cellular Mobile Environments”, Wireless Networks(3), 1997; and, D. Cox and D. Reudnick. “Increasing Channel Occupancy in Large Scale Mobile Radio Systems: Dynamic Channel Assignment”, IEEE Trans. On Vehicular Techology, 1973. In such mechanisms, each access point is able to simultaneously communicate on multiple channels. When overloaded, an access point borrows idle communication channels (essentially additional capacity) from neighboring cells. The coverage area of each access point, however, remains unchanged. Such schemes work only when an individual AP has expensive hardware and specialized software to support multiple simultaneous channels. Many AP implementations (e.g., those for IEEE 802.11-based Wireless LANs) can support only one channel—the capacity of each AP is then fixed. Implementations however do have flexibility in setting the transmission power level. In such environments, channel-borrowing schemes cannot work: load-balancing is, however, possible using the implicit approach of power control.
Other published work, for example, as found in the works of S. V. Hanly. “An Algorithm for Combined Cell-Site Selection and Power Control to Maximize Cellular Spread Spectrum Capacity”, IEEE Journal of Selected Areas in Communication, September 1995, and, J. Qiu and J. Mark. “A Dynamic Load Sharing Algorithm through Power Control in Cellular CDMA”, have discussed the use of power control by an AP while communicating with its set of attached wireless devices. The focus of the systems described in these references is to reduce the communication power between an AP and a set of already attached wireless devices to the minimum level necessary to sustain communication. Such reduction not only conserves energy, but also serves to reduce interference to neighboring cells. It is true that, during such communication, the reduction in power implies a corresponding reduction in the communication range of the access point.
It is understood that, in these prior art schemes, reduction is performed only during the communication with an attached wireless device. That is, power control is never performed to control the set of devices that can legitimately attach to this access point.
Wireless devices typically determine the set of possible APs by using intermittent beacon signals. Approaches such as described in above-referenced references to S. Tekinay and B. Jabbari entitled “Handover and Channel Assignment in Mobile Cellular Networks” IEEE Communication Magazine, November 1991 and S. Das, S Sen and R. Jayaram entitled “Dynamic load Balancing Strategy for Channel Assignment using Selective Borrowing in Cellular Mobile Environments”, Wireless Networks(3), 1997, have no notion of increasing or decreasing the power levels of these beacon signals; instead, they adjust power levels only on a per-packet basis, after a node has attached to an access point. Accordingly, systems such as described in these prior art references do not solve the traffic load balancing problem.
Thus, there exists a need for a novel traffic load-balancing solution for AP's in cellular and wireless communications networks.
To date, no prior work discusses the notion of gratuitously and dynamically increasing the power level of an access point to increase the set of wireless devices that can attach to it. That is, AP transmission power is simply not treated as a parameter of the load-balancing solution.
It would thus be highly desirable to provide a load-balancing approach that implements the concept of proactive increase or decrease of the transmission power by an AP.